Are you doing physics by any chance next year? Because it does help with 10-15% of the spesh part of vectors/mechanics/kinematics
especially if you are doing structures and materials as a detailed study.. Also methods inconjunction with spesh is pretty good because there are alot of stuff that overlaps diff,trig,intergration ...etc. however the workload for both of them combined is quite big so that might be an issue. Also further is a walk in a park if you are good at maths you probably wouldnt have to study as much, but u do have get used to a ti 84 if you are not using it already. You probably benefit from the matrices work covered in further for methods but thats about it, the rest is arithmatic/patterns/geometry . Umm also what course you interested in, spesh gives middle band bonus points for some courses where further doesnt so yea that could also be a factor. but yea its really up to you balance cons and pros and see your careers teacher/maths teacher.